1. Field of the Invention
The present invention relates to an ink-jet printer, a printing method and an ink-jet head.
2. Discussion of the Background
When a color image for advertisement or the like is printed on such a medium as a transparent window glass or a transparent film by using an ink jet printer, it is impossible to directly print an image including the color image on the medium with ink droplets jetted from an ink jet head. Usually, in such a case, a ground-color layer of, for example, white is first printed on the transparent medium with ink droplets for printing a ground-color layer jetted from an ink jet head. Then, an image is printed over the front surface of the ground-color layer with ink droplets for printing an image jetted from another ink jet head. That is, printing is performed twice on the medium, with the ground-color layer and the image printed over each other.
In this way, an image including a color image is printed on a transparent medium with the interposition of a ground-color layer, with the image printed over the ground-color layer. This is because if the image is solely and directly printed on the transparent medium, high-brightness intermediate colors of the image, such as gray other than high-density colors such as red, blue, yellow, and black, for example, are disturbed by lights infiltrating in the image through the transparent medium and thus are not precisely expressed.
Similarly, when a color image for advertisement or the like is printed on the back surface of a transparent medium, an image is printed on the back surface of the medium with ink droplets for printing an image jetted from an ink jet head, and thereafter a ground-color layer of, for example, white is printed over the back surface of the image printed on the medium. In this way, the ground-color layer is printed over the back surface of the image including a color image, which has been printed on the medium. This is also because if the image is solely and directly printed on the back surface of the transparent medium, high-brightness intermediate colors of the image, such as gray other than high-density colors such as red, blue, yellow, and black, for example, are disturbed by lights infiltrating in the image through the transparent medium and thus are not precisely expressed, when viewed through the transparent medium from the front surface side of the medium.
Further, when an image is printed by an ink jet printer on such a medium as fabric, a metal plate, or leather which is difficult to be directly and stably printed with an image including a color image, an image-fixing undercoat layer is printed on the medium with ink droplets for printing an undercoat layer jetted from an ink jet head, and thereafter an image is printed over the front surface of the undercoat layer with ink droplets for printing an image jetted from another ink jet head. That is, printing is performed twice on the medium, with the undercoat layer and the image printed over each other.
Further, when an image is printed on such a medium as a metal plate or leather which is difficult to be stably fixed and printed with an image including a color image, the image is printed on the medium with ink droplets for printing an image jetted from an ink jet head, and thereafter an overcoat layer is printed over the front surface of the image, which has been printed on the medium, with ink droplets for printing an overcoat layer jetted from another ink jet head. The overcoat layer covers and protects the front surface of the image to prevent the image from being scraped off from scrubbing or to prevent the image from having color fading under ultraviolet light.
Therefore, for printing an image including a color image on the front or back surface of a transparent medium by using an ink jet printer, the ink jet printer includes a main ink jet head for jetting ink droplets for printing an image, and also a sub ink jet head for jetting ink droplets of, for example, white for printing a ground-color layer, with the two ink jet heads aligned with each other. Thus, a ground-color layer is printed on the front surface of the transparent medium with the ink droplets jetted from the sub ink jet head, and thereafter an image is printed over the front surface of the ground-color layer with the ink droplets jetted from the main ink jet head. Alternatively, conversely, an image is printed on the back surface of the transparent medium with the ink droplets jetted from the main ink jet head, and thereafter a ground-color layer is printed over the back surface of the image, which has been printed on the medium, with the ink droplets jetted from the sub ink jet head.
Further, for printing an image on such a medium as a metal plate or leather which is difficult to be stably fixed and printed with an image including a color image, an ink jet printer includes a main ink jet head for jetting ink droplets for printing an image, and also a sub ink jet head for jetting ink droplets for printing an overcoat layer, with the two ink jet heads aligned with each other. Thus, an image is printed on the medium, which is difficult to be fixed with an image, with the ink droplets jetted from the main ink jet head, and thereafter an overcoat layer is printed over the front surface of the image, which has been printed on the medium, with the ink droplets jetted from the sub ink jet head. The overcoat layer covers and protects the front surface of the image to prevent the image from being scraped off from scrubbing.
Meanwhile, in the above-described case wherein the ink jet printer which includes the sub ink jet head in addition to the main ink jet head, with the two ink jet heads aligned with each other, is used for printing an image on a transparent medium with the interposition of a ground-color layer, for printing an image on a medium which is difficult to be directly printed with an image, with the interposition of an undercoat layer, for printing a ground-color layer over the back surface of an image printed on the medium, and for printing an overcoat layer over the front surface of an image printed on the medium, as described above, the sub ink jet head which jets the ink droplets for printing a ground-color layer or an undercoat layer is caused to perform a reciprocating movement along a line Y parallel to the horizontal direction, and the medium is sent forth and moved on a platen by sending unit in the front direction of an X direction.
Alternatively, the main ink jet head which jets the ink droplets for printing an image is caused to perform a reciprocating movement along the line Y corresponding to the horizontal direction, and the medium is sent forth and moved on the platen by the sending unit in the rear direction of the X direction. Thus, the ground-color layer, the undercoat layer, or the image is printed at a desired position on the medium. Then, after the ground-color layer or the undercoat layer has been printed on the medium, or after the image has been printed on the medium, a part of the medium placed on the platen is nipped between a grid roller and a pinch roller of the sending unit, and the grid roller is rotated in the reverse direction. Thus, the medium is moved and returned on the platen by the sending unit in the direction opposite to the X direction corresponding to the direction of sending forth the medium. Thus, the main ink jet head which jets ink for printing an image is positioned above the ground-color layer or the undercoat layer previously printed on the medium, or the sub ink jet head which jets ink for printing a ground-color layer or an overcoat layer is positioned above the image previously printed on the medium. Then, the ink droplets for printing an image are jetted from the main ink jet head, or the ink droplets for printing a ground-color layer, an undercoat layer, or an overcoat layer are jetted from the sub ink jet head.
In such a case, while the pinch roller of the sending unit is pressed to contact the front surface of the ground-color layer, the front surface of the undercoat layer, or the back surface of the image, which has been previously printed on the medium, the pinch roller is rolled and moved on the ground-color layer, the undercoat layer, or the image to return in the direction opposite to the direction of sending forth the medium. Therefore, a trace of the pinch roller is left on the ground-color layer, the undercoat layer, or the image, and thus the quality of the image printed over the surface of the ground-color layer or the undercoat layer and the image printed on the medium is substantially deteriorated.
Japanese Unexamined Patent Application Publication No. 2002-205381 discloses an ink jet printer which includes a monochromatic print unit for ejecting ink drops of one color from a plurality of nozzles arranged in an ink jet head, and an overlap printing unit for moving and resetting the ink jet head above the print start part on the surface of a recording medium printed with a specified sectional length of a drawing or character every time when the specified sectional length of a monochromatic drawing or character is printed in the X direction on the surface of the recording medium by the monochromatic print unit and printing a monochromatic diagram or a character of a different color from a previously printed drawing or character on the surface of the recording medium printed with the specified sectional length of a drawing or character by the monochromatic print unit and dried not to cause any blur. The contents of this publication are incorporated by reference in their entirety.